This invention relates to carbon-carbon composites. More particularly, the invention relates to filled materials comprising particulate fillers and a pitch matrix which may be used in the production of carbon-carbon composites. Still more particularly, this invention relates to B-staged, thermoformable compositions comprising particulate fillers and a pitch matrix, and to a method for producing preforms and shaped articles from such compositions.
Carbon-carbon composites are well known and are widely used in many applications where good mechanical properties at extreme temperatures and pressures are needed. Generally, a carbon-carbon composite comprises a heterogeneous combination of carbonaceous reinforcing material such as carbon fibers, graphite fibers, carbon particles or the like, dispersed in a carbon matrix.
Carbon-carbon composites may be made, for example, by carbonizing fabricated structures formed by impregnating a carbonaceous reinforcing material with pitch or an organic resin in liquid form.
The organic resins ordinarily used in impregnating processes are low molecular weight, soluble, often liquid resins or resin precursors known in the art as A-stage thermosetting resins, and may include, for example, epoxy resins, phenolic resins, a variety of curable acrylic resins, urea formaldehyde resins, and the like. Such resins will be advanced or further polymerized during or after the impregnation step to provide impregnated reinforcement in an incompletely cured or B-staged condition. The B-staged impregnated material or prepreg remains thermoplastic and may be laid up, wound or otherwise fabricated and further formed or shaped as desired before being finally cured into a thermoset composite article. The thermoset composite will then be carbonized or pyrolyzed in a heating step to decompose the matrix resin and form a shaped carbon-carbon composite article. The high temperatures used in the carbonizing process produce volatile, low molecular weight by-products that cause the formation of voids, blisters and other defects. Such defects may be minimized by carrying out the carbonizing step under pressure and in a mold.
Where the impregnating material is pitch, carbonizing the impregnated reinforcing material ordinarily requires the use of a mold or other container. Unlike the cured or thermoset organic resins, pitch melts, liquifies and flows on heating. A containment vessel is necessary to prevent loss of the pitch component during the carbonizing step, and may also be needed to constrain the pitch-impregnated reinforcing material to the desired shape. In addition, volatiles evolved during pitch carbonization often cause the molten pitch to foam, resulting in a porous, low density structure having voids and other defects. Prior art processes for manufacturing pitch-based composites have generally relied on applying substantial pressure during the carbonizing step, usually employing a mold or similar pressure vessel, to minimize such problems.
Typical processes for producing a carbon-carbon composite from organic resins or pitch and carbon fibers are discussed by J. Delmonte in "Technology of Carbon and Graphite Fiber Composites," Van Nostrand Reinhold Company, New York, 1981, Chapter 13.
U.S. Pat. No. 3,943,213 discloses dispersing from about 1 to about 10 weight percent of chopped carbon fibers (less than about one inch long) in a hot, fluid, pitch system, forming or casting a shaped article, and pyrolyzing and graphitizing the resulting article in its containment vessel. The patent, at Col. 2, lines 25-26, discloses carbonizing the composite at about 400.degree. C. to 650.degree. C., then graphitizing by conventional methods. Although the fibers can be mixed with the liquid pitch at or below the point at which mesophase is formed, the mixture is converted to mesophase pitch before casting the finished article, with or without applying external pressure, while the mesophase pitch is still liquid.
German Patent No. 2,714,364 relates to a process for producing a carbon-carbon composite by impregnating either chopped carbon fiber or a wound, continuous carbon fiber structure with a liquid pitch, converting the pitch to mesophase pitch, then carbonizing the mixture at a temperature of 400.degree. C. to 1,000.degree. C. while maintaining a mechanical pressure of 725 to 11,000 psi on the mixture during the carbonizing. The heat treatment is carried out under pressure to produce an infusible, densified body.
It will be apparent that the known processes for manufacturing pitch-based carbon-carbon composites do not lend themselves well to the production of complex shapes. Casting shaped articles requires the use of expensive molds and pressure equipment. Reducing foaming and void formation in composite castings is difficult, and additional impregnating and carbonizing post-treatments are frequently needed to fill voids and cracks and to increase the bulk density of the resulting composite article.
A thermoformable composition having the characteristics of a B-staged resin and comprising a thermoplastic pitch and a particulate filler would provide a substantial improvement in the carbon-carbon composite manufacturing art. Such a composition, in the form of sheet or rod or in a chopped or pelletized form, could be compression molded or hot-pressed by conventional methods to form a dense preform structure or article having the desired shape. The resulting preform could then be carbonized, with or without use of a containment vessel or mold, to provide carbon-carbon composites in shapes with otherwise difficult to attain complexity.